The present invention relates to data processing apparatuses which include preprogrammed functions, and specifically preprogrammed functions which require two integral operands. This functional capability is most often found in the so called "scientific" calculators. These calculators generally include a wide variety of functions in addition to the basic rour arithmetic operational functions of addition, substraction, multiplication and division. These machines typically include a keyboard for entering information and operational commands into the machine and a visual display for communicating an indication of the results to the operator. These machines often include preprogrammed functions which require two integral operands such as the calculation of the number of permutations of n things taken r at a time and the number of combinations of n things taken r at a time.
In the prior art the two integral operands required for these functions are entered in the normal manner of operand entry. That is, the first operand is entered, the functional command is entered and then the second operand is entered. This entry method, which corresponds generally to the manner in which an algebraic expression is entered, is termed the algebraic entry method. According to this technique two pending operand memories are required for entering these two integral operand functions. This system also requires that these integral operand functions occupy a place within the mathematical hierarchy of operation along with the other two operand functions such as addition, subtraction, multiplication and division. Because the mathematical hierarchy properly assignable to such functions as permutation and combination are more akin to those assigned to unitary operations such as the trigometric functions or logarithmic functions, this prior entry method sometimes leads to less than satisfactory performance of the desired function.